1. Field of the Invention
The present invention relates to an NMR probe for use with a high-resolution nuclear magnetic resonance (NMR) spectrometer to perform high-temperature NMR measurements.
2. Description of the Related Art
An NMR probe for high-temperature NMR measurements is an important attachment that is indispensable where researches into physical properties or in site observations of chemical reactions are made using an NMR instrument. Especially, in the field of research into supercritical fluids (where the temperature of an investigated sample needs to be maintained at a high temperature over 400° C.) and in the field of research into inorganic materials, it can be reasonably said that NMR probes for high-temperature NMR measurements are indispensable tools.
The structure of a prior-art NMR probe for high-temperature NMR measurements is shown in FIG. 1. The probe has a fluid intake port 1 for taking in a fluid such as nitrogen gas. A fluid such as nitrogen gas sent in from the fluid intake port 1 flows through a channel formed inside the NMR probe and is heated by a heater 3 mounted on the upstream side of the position of an NMR sample tube 6. Electric power for heating is supplied from an external power supply (not shown) to the heater 3 via a power-supply connector 2.
The channel for fluid is surrounded by a heat-insulating means such as a vacuum double tube 7 to supply the heated fluid into the position of the NMR sample tube 6 while maintaining the high temperature. Thus, the channel is thermally insulated from the outside. The temperature of the fluid is measured by a temperature sensor 4 such as a thermocouple at a temperature-measuring point 5 located immediately below the NMR sample tube 6. The electric power supplied to the heater 3 is controlled based on the value of the measured temperature. That is, where the temperature of the fluid is lower than a preset value, the amount of electric power supplied to the heater 3 is increased. Conversely, where the temperature of the fluid is higher than the preset value, the amount of electric power supplied to the heater 3 is reduced. The fluid such as nitrogen gas can be controlled to a desired temperature because of the structure described so far.
The heated fluid is passed through the thermally insulated channel inside the NMR probe for high-temperature NMR measurements and blown against the NMR sample tube 6. Because of heat exchange between the NMR sample tube 6 and the fluid, the tube 6 is heated to a high temperature. In order to maintain the temperature of the sample tube 6 at a high temperature of 400° C., it is necessary that the heated fluid be sufficiently thermally insulated from the outside environment by the heat-insulating means such as the vacuum double tube 7 and that a heater adapted for high-power applications be used as the heater 3 (see Japanese Patent Laid-Open No. 2002-168932).
However, as shown in FIG. 1, the prior-art NMR probe for high-temperature NMR measurements is designed such that the heated fluid is blown upward from a lower part of the NMR sample tube 6. Therefore, the bottom of the sample tube 6 is at the highest temperature. The temperature drops with going toward the top of the sample tube 6. Therefore, there is the problem that a temperature gradient is produced along the height of the NMR sample tube 6. The temperature gradient becomes steeper with increasing the set temperature of the fluid. Also, as the outside diameter of the used sample tube 6 is increased as encountered where a sample tube of a large diameter such as more than 10 mm is used, the temperature gradient tends to become steeper. Consequently, it is quite difficult to heat the whole volume of the sample within the NMR sample tube at a uniform temperature.
Especially, where the sample is a supercritical fluid, it is necessary to apply high pressure to the sample, in addition to high temperature. Accordingly, the sample tube needs to have a thick wall to withstand the high pressure. As a result, the outside diameter of the sample tube is increased. This results in a steeper temperature gradient.
If the temperature gradient becomes steeper, the supercritical fluid becomes different in nature between the top and bottom of the NMR sample tube. This creates the problem that the obtained NMR data is quite complex. To solve this problem, techniques for mounting a heater over the NMR measurement portion are proposed in the above-cited Japanese Patent Laid-Open No. 2002-168932 and Japanese Patent Laid-Open No.2001-281314.
However, in the above-described prior-art NMR probe for high-temperature NMR measurements, heat from the heater is transmitted to the measured sample via a fluid such as nitrogen gas. Therefore, large-sized fluid supply equipment such as a nitrogen gas cylinder is necessary. In addition, only a small portion of the thermal energy given to the fluid is used to increase the temperature of the measured sample. Most of the thermal energy has been discarded into the atmosphere together with the fluid. For this reason, most of the thermal energy is wasted. In this way, the energy efficiency has been very low.
Additionally, in the prior-art NMR probe for high-temperature NMR measurements, a high-power heater has been used because of low thermal efficiency. Therefore, there is the possibility that a large amount of heat produced from the heater adversely affects electronic parts located around the NMR probe for high-temperature NMR measurements. Accordingly, to avoid this problem, strict countermeasures for thermal insulation and cooling have been essential.
A technique for transmitting the heat from heating means disposed respectively above and below a sample tube to a sample tube via a sample coil for detecting the NMR signal from the sample and via a coil bobbin supporting the sample coil is also proposed (see Japanese Patent Laid-Open No. 2002-196056). However, there is a danger that certain limitations will be produced if a function of transmitting heat to the sample coil and coil bobbin is added while satisfying the essential functions of the sample coil and coil bobbin.